Electronic device encapsulated directly on a substrate

ABSTRACT

A method and apparatus for encapsulating one or more small electronic devices, including for example light-emitting diodes, mounted directly on a substrate by providing a three-dimensional formation on the substrate adjacent to the device and injection molding a thermoplastic encapsulating material to cover the device and extend over the three-dimensional formation on the substrate and wherein the encapsulating material mechanically bonds to the three-dimensional formation. A gate plate for use in injection molding a thermoplastic encapsulating material over a small electronic device mounted directly on a substrate including a cavity formed to enclose the device, and a short gate formed entirely within the gate plate having an input for receiving an encapsulating material and an output communicating with the cavity. One or more encapsulated electrical devices mounted directly on a substrate, including, for example, an LED or an array of LEDs, wherein the device is fully and individually encapsulated by an encapsulating material which is injection molded onto and mechanically bonded to the substrate.

This is division of U.S. Application Ser. No. 08/763,538, filed Dec. 10,1996 now U.S. Pat. No. 5,833,903.

BACKGROUND

The invention relates to plastic encapsulation of electronic devices,and more specifically, to injection molding an encapsulation for anelectronic device directly onto a substrate such as a printed circuitboard.

It is well known that electronic devices are sensitive to theenvironment and that exposure to normal atmospheric conditions maydegrade or ruin them entirely. Accordingly, it is the current practiceto protect electronic devices from environmental/atmospheric exposure bysealing them within a protective enclosure, commonly made of anon-electrically conducting material such as a plastic resin, with aninterfacing means, such as pins, to allow connection of the devices to alarger electronic circuit or other devices. Simple devices such asresistors, capacitors, diodes and the like, as well as more complexsemiconductor devices, or chips, are commonly packaged in this manner.

It is common practice to interface such an encapsulated device withother devices mounted on a supporting substrate by, for example,inserting its interface pins into a corresponding socket mounted on thesubstrate. These other devices are similarly mounted and connected toeach other with wires, or with traces in the case where the substrate isa printed circuit board.

This practice of encapsulation suffers from a number of drawbacks.Generally, the equipment and materials necessary to accomplish theencapsulation must be located outside of the clean room environmentwhere the device itself is manufactured, and the encapsulation musttherefore be performed as a separate manufacturing step. Theencapsulating process is also expensive. Further, the plastic packagesthemselves, with the required interface means, significantly increasethe size of the device, thereby requiring a larger area, or more realestate, for their incorporation in another device or circuit.

Alternatively, it is known that certain electronic devices may bemounted to a substrate such as a printed circuit board, typically withgold wire connections, and encapsulated by a liquid resin that is handcast over the device on the substrate. This procedure is not preferredbecause it is expensive, time-consuming, difficult to accurately placethe cast material over the device, and provides poor adhesion of thecast material to the substrate. Further, the gold wire connections tothe device are very delicate and are easily disconnected during thecasting process.

Another prior-art method for encapsulating devices mounted onto asubstrate by gold wires is the so-called "transfer molding" method.Transfer molding is a process by which a thermosetting material iscaused to flow into a cavity formed by the cooperation of a mold and acavity plate. The material enters the cavity through so-called "side"gates which are also formed by the space between the cavity plate andthe mold. This method is an improvement over the hand-castingencapsulation method because it allows multiple devices to beencapsulated at the same time in one production cycle and it allows forsomewhat more accurate placement and size of the resulting encapsulatedpackage.

Transfer molding, however, itself suffers from a number of significantdrawbacks which are eliminated by the present invention. Initially,transfer molding techniques of encapsulating electronic devices arelimited to use of thermosetting materials which have a low viscosity.Such a material is necessary to prevent damage to the delicateconnections of the device to the substrate during the molding process.This same danger requires that the encapsulating material be forced intothe mold at low pressure. Use of a low viscosity thermoset results inthe need for an expensive mold apparatus which must be constructed withvery high tolerances to prevent leakage of the encapsulating material.Even the most expensive molds, however, exhibit some leakage in the areaof the gate and device connections which must be removed by additionalprocess steps after molding, thereby increasing cycle times.

Use of a thermoset, which cures by a chemical process, also results inlong cycle times, on the order of 5 to 15 minutes, which increasesproduction costs. Thermoset materials themselves are expensive due, inpart, to the inability to reuse excess encapsulating material resultingfrom the molding process after the material has cured.

Use of a low viscosity thermoset at low insertion pressure also resultsin the need for large side entry gates for the encapsulating material.The large side gates make transfer molding impractical for small devicesbecause the size of the gate limits the size of the cavity. The gatingused in transfer molding techniques adds further limitations to theplacement and configuration of the devices to be encapsulated because itrequires the devices to be near an edge of the substrate to which it isto be bonded. Typical transfer molding applications therefore involvedevices which are mounted in a linear arrangement on a substrate withthe use of strip-like carriers, or "lead frames," for the devices.Transfer molding with thermosets is also not useful with small devicesbecause the thermosetting material requires substantial surface area incontact with the substrate in order to adhere sufficiently to hold thedevice and encapsulate to the substrate with a chemical or adhesivebond.

On the other hand, use of higher viscosity thermoplastic materials isnot practical in transfer molding because it requires higher pressuresthat may damage the device connections and may result in additionalleakage of the encapsulating material. Further it is difficult in atransfer molding apparatus to maintain the high temperatures required toallow a thermoplastic material to properly flow.

Attempts have been made to solve the problems with prior-artencapsulating methods by use of injection molding. Prior art injectionmolding methods, however, suffered from similar drawbacks. Althoughhigher pressures may be used with injection molding and thus would allowuse of thermoplastic materials, the injection process would damage thedelicate device connections. In addition, prior art injection moldingmethods and devices were not useful for small devices because thesmaller gating necessitated by smaller cavities had a tendency to clogwith the thermoplastic material and this material exhibited pooradhesion resulting in devices being separated from the substrate.

SUMMARY

In general, in one aspect, the invention features a method ofencapsulating a small electronic device mounted directly on a substrateby providing a three-dimensional formation on the substrate adjacent tothe device and injection molding a thermoplastic encapsulating materialto cover the device and extend over the three-dimensional formation onthe substrate and wherein the encapsulating material mechanically bondsto the three-dimensional formation. In another aspect, the inventionfeatures a method of encapsulating a light emitting diode (LED) mounteddirectly on a substrate by providing a hole through the substrateadjacent to the LED and injection molding a light-transmissivethermoplastic encapsulating material to cover the LED and fill the hole.In another aspect the invention features a method of encapsulating a setof LEDs mounted directly to a printed circuit board (PCB) and arrangedto form an alphanumeric display by providing a hole through the PCBadjacent to each of the LEDs, injection molding a light-transmissivethermoplastic around each of the LEDs and wherein each of the LEDs isseparately encapsulated in a package that is shaped to focus and reflectlight from the LED and is mechanically bonded to the PCB. In a furtheraspect, the invention features a method of encapsulating a plurality ofsmall electronic devices mounted directly on a substrate in closeproximity to one another comprising providing a three-dimensionalformation on the substrate adjacent to each device, injection molding athermoplastic encapsulating material to individually cover each deviceand wherein the encapsulating material mechanically bonds to thethree-dimensional formation on the substrate.

In another aspect the invention features a mold for injection molding athermoplastic encapsulating material over a small electronic devicemounted directly on a substrate, the mold comprising a base member, atop member including an inlet, and a gate plate including a short gatehaving an input communicating with the inlet and an output communicatingwith a cavity, and wherein the gate is formed entirely within the gateplate. In another aspect the invention features a gate plate for use ininjection molding a thermoplastic encapsulating material over a smallelectronic device mounted directly on a substrate comprising a cavityformed to enclose the device, and a short gate having an input forreceiving an encapsulating material and an output communicating with thecavity, and wherein the gate is formed entirely within the gate plate.In a further aspect, the invention features a gate plate for use ininjection molding a light-transmissive thermoplastic encapsulatingmaterial over a set of LEDs mounted directly on a substrate comprising aset of cavities arranged in the form of an alphanumeric display, eachcavity formed to enclose one LED and extend over its adjacent hole and aset of short conical gates, each having an input for receiving anencapsulating material and an output communicating with a cavity, andwherein the gates are formed entirely within the gate plate.

In another aspect, the invention features an encapsulated electricaldevice mounted directly on a substrate wherein the device is fullyencapsulated by an encapsulating material which is injection molded ontoand mechanically bonded to the substrate. In a further aspect, theinvention features an alphanumerical display comprising a set of LEDsmounted directly to a PCB with holes through the PCB adjacent to each ofthe LEDs and arranged to form a display and wherein each of the LEDs isseparately encapsulated in a package of light-transmissive material thathas been molded onto and mechanically bonded to the PCB.

Preferred embodiments of the invention include one or more of thefollowing features. A three-dimensional feature on the substrateadjacent to the device to be encapsulated where the three-dimensionalfeature is optionally a hole, a raised member extending above thesurface of or a groove extending below the surface of the substrate. Asubstrate comprising a printed circuit board. An encapsulating materialcomprised of a thermoplastic resin chosen from among the groups ofpolycarbonates and acrylics. A method wherein, after injection, theencapsulating material is located on less than the entire surface of thesubstrate. Encapsulating a light emitting diode in a light-transmissiveencapsulating material. Encapsulating on a PCB a light emitting diodeusing a lozenge shaped hole adjacent to the LED and wherein, afterencapsulation, the encapsulating material on the side of the PCBopposite from the light emitting diode is substantially flat and flushwith the surface of the printed circuit board. Encapsulating a pluralityof LEDs on a PCB in the form of an alphanumeric display including, forexample, a seven segment display. Encapsulating a plurality of devicesarranged in a non-uniform arrangement on a substrate and wherein one ormore of the devices are located away from the edges of the substrate. Agate plate having a substantially conical gate. A gate having a crosssectional area that is reduced from the input to the output to form aregion of relatively reduced strength in hardened encapsulatingmaterial, whereby the material is caused to break off near to the outputof the gate when the mold is opened. A gate having sidewalls that areinclined about 15 degrees over the length of the gate from the input tothe output. A gate plate about 0.250 inches thick. A gate plateincluding a distribution runner connected to the gate. A gate about0.065 inches long. A set of cavities arranged in the form of analphanumeric display. A set of cavities arranged in the form of aseven-segment display. A cavity shaped to form a package ofencapsulating material that focuses and reflects light from a lightemitting diode out of an adjacent hole in the substrate. A cavity havinga base opposite from the output of the gate, which base is substantiallytriangular with rounded corners and which cavity is substantiallyrounded above its base. An encapsulated electrical device mounteddirectly on a substrate wherein the device is fully encapsulated by anencapsulating material. An alphanumerical display comprising an array oflight emitting diodes separately encapsulated in a package oflight-transmissive thermoplastic material.

Among the advantages of the invention are one or more of the following.The invention eliminates the need for separate, expensive andtime-consuming encapsulation of electronic devices. The inventioneliminates the need for pin interfaces on circuit boards. The inventionreduces the amount of space required on a circuit board for a givenelectronic device. In one aspect, the invention provides an injectionmolding method where the encapsulating material hardens quickly by lossof heat, without leakage of material, thereby increasing productionspeed and capacity. The invention increases production speed byeliminating the need to mill excess encapsulating material aftermolding. The invention allows for economical reuse and recycling of athermoplastic encapsulating material. In another aspect, the inventionprovides apparatus for injection molding that is efficient andeconomical. In another aspect, the invention provides an injection moldwith a thin gate plate that is separate from the larger top and bottomplates of the mold and that may be designed to be constructed ofrelatively inexpensive material and therefore economically replaced whenworn. The invention further provides an injection molding apparatus thatallows placement of an encapsulating material in any location and in anyconfiguration on a substrate. In another aspect the invention providesan injection molding apparatus that allows many small devices to beindividually encapsulated in close proximity to one another. Theinvention alleviates the potential problems of wires or traces beingtorn or disconnected from the device and prevents the device from beingseparated from the substrate by making use of small devices that may bemounted directly to the substrate without wires and by mechanicallybonding the encapsulated material to the substrate. In another aspect,the invention quickly and economically encapsulates a light emittingdiode on a printed circuit board using a minimum of real estate. Theinvention further provides an alphanumeric display economicallyconstructed on a printed circuit board by injection molding a separateencapsulation around each of an array of LEDs mounted directly to theboard. Other features and advantages of the invention will becomeapparent from the following description and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a mold for injection molding a package ofan encapsulating material for an electronic device onto a substrate.

FIG. 2A is an exploded view of the mold of FIG. 1 with a substrate inposition to be injection molded.

FIG. 2B is a detailed view of the substrate of FIG. 2A.

FIG. 2C is a more detailed view of a single LED mounted to the substrateof FIG. 2B.

FIG. 2D is a detailed view of the substrate of FIG. 2B including themolded package.

FIG. 2E is a section view of the substrate of FIG. 2D.

FIG. 2F is a view of an alternate embodiment of the substrate of FIG. 2Dwith a raised member on the substrate instead of a hole.

FIG. 2G is a sectional view of the mold of FIG. 2A.

FIG. 2H is an enlarged view of the gate and cavity of the mold of FIG.2G.

FIG. 2I is a view of the top of the gate plate of the mold of FIG. 2A.

FIG. 2J is a view of the bottom of the gate plate of the mold of FIG.2A.

FIG. 2K is a detailed view of the cavities and gates of the gate plateof FIG. 2I.

FIG. 2L illustrates the removal of excess encapsulating material.

FIG. 2M is an enlarged view of the gate and cavity of the mold of FIG.2G shown in place above the substrate of FIG. 2D after injection andafter the removal of excess encapsulating material.

FIG. 2N is an exploded view of the mold of FIG. 2A with the packagemolded to the substrate.

FIG. 3 is a diagram of the mold of FIG. 1 in place in an injectionmolding apparatus.

DETAILED DESCRIPTION

Referring to FIG. 1, mold 100 includes top plate 10, gate plate 20 andbase plate 40. Top plate 10 includes inlet 11 that provides a path forinjecting an encapsulating material into mold 100. Gate plate 20includes gates 21, distribution runner 25 and ejection holes 26. Gates21 communicate with inlet 11 through optional distribution runner 25.Alternatively, gates 21 may communicate directly with inlet 11. However,use of distribution runner 25 improves the flow of an encapsulatingmaterial to gates 21 and facilitates the creation of shorter gates. Baseplate 40 includes ejection pins 41 and locating pins 42.

Referring to FIG. 2A, substrate 30 is positioned within mold 100 bybeing placed on base plate 40 so that locating holes 33 fit overlocating pins 42. Gate plate 20 is placed on top of substrate 30 and topplate 10 is placed on top of gate plate 20. (Substrate 30 is not part ofmold 100).

Referring to FIG. 2B, substrate 30 is a printed circuit board as is wellknown, and light emitting diodes ("LEDs") 32 are mounted directly tosubstrate 30. Small electronic devices such as LEDs 32 may be mounteddirectly to substrates without the need for delicate gold wireconnections. The connections may be made "pad-to-pad", i.e., betweenconductive surfaces on the upper surface of the substrate and on thebottom surface of the device itself. Mounting of the device directly tothe substrate without delicate gold wires eliminates the prior artproblem of damage to these connections which occurred during the moldingof an encapsulation.

Substrate 30, additionally includes a three-dimensional formationadjacent to each LED 32. As shown, this three-dimensional formationcomprises holes 31 through substrate 30. In addition to a printedcircuit board, substrate 30 may, alternatively, be any suitableelectrically insulating material and may be, for example, a bread-board,a plastic plate, a piece of glass, a coated metal plate or othermounting surface. In the specific LED application shown, holes 31 arelozenge shaped and arranged to form the familiar seven-segmentarrangement of numeric displays. Each LED 32 has a corresponding hole 31adjacent to it. Other arrangements of holes and LEDs may be contemplatedthat may be used to form more generalized alphanumeric displays. Forexample, a simple rectangular array or grid of LEDs may be used togenerate a wide variety of characters and the implementation of suchalphanumeric displays is well known.

Referring to FIG. 2C LED 32 is connected to trace 38 which electricallyconnects LED 32 so LED 32 may be used as part of a completed circuit.Each LED 32 is similarly connected to its own trace 38 formed insubstrate 30 which is a printed circuit board.

Referring to FIG. 2D, after completion of the encapsulation process, apackage of hardened encapsulating material 34 completely encloses LED32. The package of encapsulating material 34 also fills hole 31. Holes31 promote the bonding of package 34 to substrate 30. As it hardens, theencapsulating material shrinks and forms a mechanical bond with thesubstrate at hole 31. This mechanical bonding allows for very smalldevices to be encapsulated and strongly held to the substrate. Althoughshown of a shape and size to form a seven-segment display, when devicesother than LEDs are being encapsulated, holes 31 need only be ofsufficient number, size and placement to insure adequate bonding of theencapsulating material. The specific number, size and placement will bedetermined, in turn, by the size and shape of the device beingencapsulated.

FIG. 2D shows that one set of seven LEDs has been encapsulated. However,the encapsulation of a single device, several devices, or all thedevices on substrate 30 may be accomplished in one injection cycle. Inpractice, a large number of sets of LED 32, on multiple substrates, willbe encapsulated in one injection cycle.

Substrate 30 may contain any kind of small electronic device to beencapsulated. LED 32 is shown by way of example only and the disclosedmethod and apparatus may be used to quickly and economically injectionmold a package of any shape around any kind of device that may bedirectly connected to a substrate. For example, simple devices such asresisters or capacitors as well as more complex devices such as RandomAccess Memory ("RAM"), microprocessors, or any other electronic devicesmay be encapsulated in a similar fashion. The disclosed method andapparatus are especially useful in encapsulating a number of very smallelectronic devices (on the order of approximately 12/1000 inch indiameter and larger) that are to be placed in close proximity to oneanother on the same substrate (within approximately 12/1000 inch fromone another and further apart).

The encapsulation of the device is advantageously accomplished bymechanically bonding the encapsulating material directly to thesubstrate. In one example shown in FIG. 2A, this bonding is accomplishedby the use of holes 31 in substrate 30 which retain the encapsulatingmaterial as it shrinks during hardening and promotes the formation of amechanical bond. Holes 31 may be of any shape and may be created throughany method, for example, with a punch, a laser, or by etching, millingor routing the substrate. An added advantage to using holes 31 insubstrate 30 to aid the encapsulation of LED 32, as contrasted withother three-dimensional formations discussed below, is that the LED's 32light may be transmitted through the holes 31 from the LED 32 mounted onthe back side of substrate 30.

This mechanical bonding of the encapsulating material to the substratemay also be promoted by other three-dimensional formations on thesubstrate such as, for example, raised members extending above thesurface of the substrate that "grip" the encapsulating material as itshrinks during hardening, or recesses extending below the surface of thesubstrate that retain the encapsulating material in a manner similar tothat of holes 31. The mechanical bonding allows for very small devicesto be encapsulated in small packages wherein only a small contactsurface exists between the encapsulating material and the substrate.Such a small contact surface, on the other hand, prevents the formationof an adhesive or chemical bond which has adequate strength alone tohold a device and package to the substrate.

Referring to FIG. 2E, the package of hardened encapsulating material 34can be more easily seen to completely enclose LED 32. The package ofhardened encapsulating material 34 also fills in hole 31 and forms asurface flush with the bottom surface 36 of substrate 30. The shape ofthe package will be determined by the shape of the cavity of gate plate20. The configuration of these cavities will be discussed in more detailbelow. As shown in FIG. 2D, package 34 is substantially triangular withrounded corners at the surface of substrate 30 and is rounded as itrises above the surface to form a roughly semi-spherical shape. Theshape and composition of the package of hardened encapsulating material34 also acts to focus and reflect light transmitted from LED 32 out hole31. In an application using an LED, this configuration allows the bottomsurface 36 of substrate 30 to be the "face" of the device and provide adesireable flat display.

Referring to FIG. 2F, the three-dimensional formation may be embodied asa raised member 39 on substrate 30 adjacent to LED 32. The package ofhardened encapsulating material 34 forms a mechanical bond withsubstrate 30 and raised member 39 in a manner similar to that describedabove when hole 31 is provided.

Referring to FIG. 2G, a section view of mold 100, substrate 30 ispositioned on top of base plate 40 within closed mold 100. Gate plate 20includes gate 21 and cavity 24. Gate 21 communicates with inlet 11through distribution runner 25. Cavity 24 communicates with gate 21.Gate plate 20 is positioned so that ejection holes 26 are located overejection pins 41 of base plate 40. Ejection pins 41 are spring-loadedand, when extended, extend completely through ejection holes 26 andabove gate plate 20. When mold 100 is closed, ejection pins 41 arecompressed downward by the weight of top plate 10.

Referring to FIG. 2H, as shown, gate 21 is conical in shape and includesan input 22 and an output 23. Input 22 communicates with inlet 11 of topplate 10 through distribution runner 25. Output 23 communicates withcavity 24. The cross-sectional area of gate 21 decreases from input 22to output 23. This reduction of cross-sectional area results in a regionof reduced strength in the hardened encapsulating material near output23. In one embodiment, gate 21 has a substantially round input 22 with across-sectional area of about 0.000615 square inches and a substantiallyround output 23 with a cross-sectional area of about 0.000113 squareinches, and is about 0.065 inches long, which provides the intendedregion of reduced strength near output 23 in a thermoplasticencapsulating material such as Lexan™, a polycarbonate available fromGeneral Electric. Alternatively stated, a gate having side walls 28 thatare inclined about 15 degrees over their length from the inlet to theoutlet will provide the desired shape.

Referring again to FIG. 2G, in operation, mold 100 is closed withsubstrate 30 positioned inside. Substrate 30 rests on rigid base plate40. An encapsulating material (not shown) is injected into mold 100through inlet 11 in top plate 10. The encapsulating material may be anythermoplastic resin, such as, for example, polycarbonate or acrylic. Useof thermoplastic resins is advantageous because they harden by loss ofheat typically in a few seconds and thus provide for very short cycletimes. Further, excess thermoplastic material not forming part of theencapsulating package may be recycled and reused simply by reheating. Inaddition, the shrinking of a thermoplastic as it cools allows for theformation of a strong mechanical bond to the substrate as discussedabove.

When injection molding a package particularly around a light emittingdevice, such as a LED, or light receiving device, such as a photodiode,the encapsulating material must be sufficiently transparent. A varietyof transparent and translucent plastic resins are available that meetthe required characteristics. A polycarbonate resin, such as Lexan™, forexample, available from General Electric, is suitable for encapsulatingan LED.

Referring again to FIGS. 2G and 2H, the injected encapsulating materialpasses through inlet 11 into distribution runner 25 and from therethrough input 22 into gate 21 and through output 23 to cavity 24. Enoughencapsulating material is injected to completely fill cavity 24, gate21, distribution runner 25 and part of inlet 11. Each cavity 24 isformed so that the encapsulating material flows in and around acorresponding hole 31 in substrate 30. The high viscosity of thethermoplastic encapsulating material prevents it from leaking beyond theboundaries formed by cavity 24 and hole 31. In addition, the pressure ofthe mold is sufficient to form a seal between the substrate 30 and baseplate 40 and cavity 24 and substrate 30 to contain the injectedthermoplastic. Once fully injected, the encapsulating material isallowed to harden. This hardening may be accomplished through loss ofheat in a matter of seconds when the appropriate thermoplastic resin ischosen as the encapsulating material.

Referring to FIG. 2I, the top portion of gate plate 20 includes gates21, distribution runner 25 and ejection holes 26. Referring to FIG. 2J,the bottom portion of gate plate 20 further includes cavities 24 andlocator holes 27. When positioned in mold 100, locator holes 27 fit overlocator pins 42 in base plate 40.

To encapsulate a small device, one requires a small cavity 24 and smallgate 21. With very small gates, the gate tends to clog or freeze off,preventing the flow of encapsulating material. This tendency may beovercome by making the length of gate 21 short and locating gate 21close to cavity 24. To facilitate a short gate 21, gate plate 20 is ofthin construction relative to the overall height of mold 100. It hasbeen found that a gate plate 20 of about 0.25 inches in thickness issuitable for use in a method and apparatus for injection molding apackage around an LED. The addition of runner 25 in the top of gateplate 20 allows the length of gate 21 to be further reduced. A gate 21of about 0.065 inches in length from input to output is well suited foruse in encapsulating LEDs. The shorter gate provides an additionaladvantage in that the thermoplastic material may be maintained at ahigher temperature because the short distance from the heating elementsto the cavity prevents substantial cooling. The higher temperatureresults in a thermoplastic with a lower viscosity which improves theinjection process further and results in less damage to delicate devicesand connections.

The small size of gates 21 and cavities 24 and their proximity to oneanother also results in a gate plate that may wear significantly withrepeated use, eventually allowing leakage between the cavity segments.Use of a thin gate plate 20 that is separate from the larger top andbottom plates (10 and 40) of mold 100, however, allows design of areplaceable gate plate which may be constructed of relativelyinexpensive material relative to the other components of mold 100.

Referring to FIG. 2K, the shape of cavity 24 and gate 21 may be seen indetail. The shape of cavity 24 is chosen to provide a package ofhardened encapsulating material (34 of FIG. 2D) that completely enclosesLED 32 and fills hole 31. Cavity 24 may, however, be of any shape, andthe shape will depend on the particular device being enclosed. Thecavity may, for example, be substantially rectangular, or round, ortrapezoidal at its base and taper to the output of its correspondinggate which itself may be, for example, round, square, triangular,rectangular, or oval. As shown, gate 21 is conical in shape with asubstantially round input and substantially round output. However, gate21 also may be of any shape and may, for example, be pyramid-shaped,substantially rectangular in cross-section, or the like.

A further advantage to the use of gate plate 20 is the versatileplacement of gates 21 and cavities 24. Because they are formed entirelyin gate plate 20, gates 21 and cavities 24 may be located anywhere ingate plate 20. Further, all of the gates may be fed from the samedistribution runner which may be located in gate plate 20 or in topplate 10. Accordingly, the devices to be encapsulated may be located inany configuration on substrate 30. The need for a lead frame to hold thedevices is thus eliminated. These devices may even be located near oraround other devices that have previously been encapsulated or mountedto substrate 30 by providing cut outs in gate plate 20 to accommodatethese other devices. The small size of gate 21 and cavity 24 andversatility of replaceable gate plate 20, allow for very accurateplacement of an encapsulating material and thus allow for a large numberof small devices to be individually encapsulated in close proximity toone another and in any location or arrangement on a substrate.

Referring to FIG. 2B, once the encapsulating material has hardened, topplate 10 is removed from gate plate 20. Ejection pins 41 areautomatically released by the removal of the weight of top plate 10 andspring upward, ejecting the excess hardened encapsulating material 50off of gate plate 20. The excess hardened encapsulating material 50breaks off near output 23 (FIG. 2H) under the pressure from ejectionpins 41. Thus, substantially all of the excess hardened encapsulatingmaterial outside of cavity 24 is automatically removed when the mold isopened, eliminating the need for any additional milling or polishingstep. Excess hardened encapsulating material 50 may be recycled by beingreheated and then used to encapsulate additional devices.

Referring to FIG. 2M, cavity 24 encloses LED 32 and its adjacent hole31. As shown in FIG. 2M and discussed above, after injection, theencapsulating material fills cavity 24 and hole 31 and fully enclosesLED 32. Also as shown in FIG. 2M and discussed above, the hardenedencapsulating material breaks off near output 23 of gate 21 when mold100 is opened.

Referring to FIG. 2N, mold 100 is further opened by removing gate plate20 and substrate 30 is thereafter removed from mold 100. The devices onsubstrate 30 are encapsulated by the hardened encapsulating material asshow in FIG. 2D and discussed above.

Referring to FIG. 3, mold 100 is contained within injection moldingapparatus 200. Injection molding apparatus 200 is of a type well-knownin the art which is commercially available, for example, from Arburg,Inc. of Newington, Conn. Injection molding apparatus 200 generallyincludes hopper 201, screw 203, barrel 205, nozzle 207 and clamp 209. Amold, such as for example, mold 100, is secured under pressure in clamp209. In normal operation, the raw encapsulating material (not shown) tobe injected is placed in hopper 201 and a portion of this materialenters barrel 205. Screw 203 rotates inside barrel 205. This rotationworks the encapsulating material into the proper molten state throughfriction and the application of heat. The rotation of screw 203 alsoworks the encapsulating material through barrel 205 toward nozzle 207.Once the encapsulating material is in the proper state, screw 203 istranslated along barrel 205 applying pressure to the encapsulatingmaterial and forcing it through nozzle 207 into mold 100.

The present invention has been described in terms of specificembodiments. The invention, however, is not limited to these specificembodiments. Rather, the scope of the invention is defined by thefollowing claims, and other embodiments are within the scope of theclaims.

What is claimed is:
 1. An alphanumerical display, comprising:a set oflight emitting diodes mounted directly to a printed circuit board; and ahole through the printed circuit board adjacent to each of the lightemitting diodes, the light emitting diodes and holes being arranged toform a display; wherein each of the light emitting diodes is separatelyencapsulated in a package of light-transmissive thermoplastic materialthat has been molded onto and mechanically bonded to the printed circuitboard.
 2. The display of claim 1, wherein the package of thermoplasticmaterial has been injection molded onto the printed circuit board. 3.The display of claim 1, wherein the package is shaped to focus andreflect light from the light emitting diode out its adjacent hole. 4.The display of claim 1, wherein each hole is completely filled with theencapsulating material such that the encapsulating material on the sideof the printed circuit board opposite from the light emitting diodes issubstantially flat and flush with the surface of the printed circuitboard.
 5. The display of claim 1, wherein the holes are lozenge shapedand arranged to form a seven-segment display.